Release liners are films or papers used for the release from adhesives, adhesive laminate constructions, or mastics in industrial operations. The term release liner is also used for films and papers that are used to cover and subsequently release from various objects, materials or parts, such as in molding operations or when handing certain types of materials. However, even though they are viable for this particular application, these types of release liners are used quite differently from those liners used in covering adhesives. Consequently, release liners used to cover various parts and objects, as opposed to adhesives, have very different performance criteria, which are generally much less stringent than those criteria applicable to liners used to cover adhesives. For example, many of the release liners used to cover various parts and objects are not sensitive to migration or adhesive release levels, as the liners do not contact an adhesive on the part or object to which they are secured.
With respect to those release liners used to cover and protect adhesives, “release level” and “release profile” are collective terms used to describe the behavior of the adhesive and release liner assessed by a number of test criteria used in measuring the tension needed to pull an adhesive away from the release liner. Most release liners must have some tension or adherence to the adhesive against which they are applied to keep the liner in position over the adhesive, and yet should be able to be pulled off of the adhesive at the correct speed and force to function properly. The release tension profile for a release liner is a very complex phenomenon.
For liners that are to be used in adhesive constructions, the release liner industry commonly uses silicone release coatings applied over polymeric film substrates to generate liner stock. Polyethylene terepthalate, polyethylene and polypropylene are common polymer film substrates for these liners, preferred grades of which are commonly known in the art. To coat these polymeric films with the silicone coating, the polymeric film must normally be corona-treated or ozone treated after it has been formed in an extrusion process to facilitate the bonding of the silicone coating to the substrate. The polymeric film is then coated with the silicone composition and cured in a suitable manner before it can be used as a release liner in a laminate assembly. Common curing methods include subjecting the silicone coating to infrared radiation, electron beam radiation, UV radiation or heated forced air to produce a dry stable silicone coated surface that will function to control the release of adhesives.
In addition to the silicone coating and curing steps being highly capital cost intensive with high operation and maintenance costs, one significant issue with the process of forming release liners in this manner is that the step of coating and subsequently curing the silicone composition onto the polymeric film introduces a wide array of variables that can detrimentally affect the proper adhesion and the correct release levels of the final film. More particularly, the release liner must be formed with the polymeric substrate and silicone coating to have a good subsequent release, i.e., the adhesive covered by the liner must have good adhesive strength after the laminate including the adhesive has been aged and released from the liner. A common problem that arises in the use of liners including the silicone coating applied in this manner is the migration of loose silicone fluid, usually in the form of uncured silicone polymer or residual oils and process fluids, from the liner onto the adhesive. As a result of this migration, the re-adhesion of the adhesive to an object other than the liner is significantly reduced.
The prior art includes a number of examples of silicone materials compounded into thermoplastic resins that produce a variety of properties. For example, U.S. Pat. Nos. 5,708,084 and 5,708,085 teach polyolefin compositions which can be extruded at relatively high rates to provide extrudate having increased hydrophobicity, said compositions comprising (A) 100 parts by weight of a thermoplastic resin; (B) 1 to 5 parts by weight of an interactive diorganopolysiloxane process aid, said diorganopolysiloxane having a number average molecular weight of at least 10,000. However, in these references, the description includes discussion of how the siloxane migrates to the surface of the plastic material, creating the desirable properties for the composition disclosed ion those references, which is directly opposite to the requirements for a release liner to be utilized on adhesives.
In addition, U.S. Pat. No. 6,080,489 discloses such a composition that exhibits surprisingly improved properties, such as having reduced coefficient of friction, consistent coefficient of friction over time, increased hydrophobicity and improved abrasion resistance. The composition includes (A) 100 parts by weight of a thermoplastic resin; (B) at least 0.5 parts by weight of a siloxane blend having both a high molecular weight (Mw) siloxane and a low Mw siloxane, where the high Mw siloxane has a viscosity greater then the low Mw siloxane. The blend of slioxanes disclosed provides the increased beneficial characteristics for the resulting products formed from the material. However, as with the previous references, other than the variation of the composition of the siloxane blend, there is nothing disclosed in the composition that effectively prevents the siloxanes from migrating to the surface of the product.
As a result, with the compositions disclosed in these references, migration of the composition may still occur because there is nothing to tether it into the polymer matrix. For this reason, the silicone additive concentrates disclosed in these references are not suitable for and have not found widespread use in commercial release liner films.
In U.S. Pat. No. 7,105,233, this reference teaches the use of organo-modified siloxanes for use as a surface modifier additive for polyolefin films which can be employed as a release liner. However, this composition also suffers from potential migration of the siloxane to the surface of the film because the siloxane is only embedded into the composition through the use of mechanical shearing forces prior to an extrusion process, and is not intertwined or otherwise bound in any manner into the system. Thus, the siloxane additive can migrate and subsequently interfere with re-adhesion of an adhesive. Additionally, the siloxanes of this invention contain Si—O—C bonds which are susceptible to hydrolytic degradation, further reducing adhesive performance stability over time.
As a result, each of these alternative silicone additive compounds has certain problems associated with it, for example, difficulties in adequately locking or binding the particular silicone compound in the polymer forming the liner, the costs of the particular additive compound and the chemical complexity of the compounds and the consequent potential interactions, i.e., migration, of the compound and other components of the release liner or the adhesive to which the liner and silicone compound are applied.
Other prior art examples include EP-A 254 050, which describes the production of a release liner by extruded application of a release agent comprising a organopolysiloxane compound that has been reacted with hydrocarbon compounds, and WO 91/15538 that describes release films comprising a base polymer and additive formed with a hard segment polymer to determine the compatibility with the base polymer, and a soft segment polymer which can be a polydiorganosiloxane polymer. However, neither of the materials disclosed in these references are capable of producing a commercially effective release liner.
Therefore, it is desirable to develop a siloxane additive for use in the formation of a release liner that does not migrate from the release liner onto the adhesive to detrimentally affect the release properties and the adhesion properties of the adhesive over which the release liner is applied. It is also desirable to develop a silicone-based additive having these benefits that is also economically feasible and easy to produce.